The present invention relates to a process for producing olefins from natural gas.
Natural gas is an abundant fossil raw material which is essentially composed of methane, the present proven reserves of which, about 10.sup.14 m.sup.3 ", represent about 50 years of world consumption. Gas fields often contain considerable amounts of ethane, propane, other higher alkanes, as well as other constituents such as H.sub.2 O, CO.sub.2, N.sub.2, H.sub.2 S, and He. Most of the propane and of the other higher alkanes associated with natural gas are liquefied and called LPG (liquefied petroleum gas). In helium-rich fields (generally more than 0.3% by volume), the helium is separated because of its high commercial value. The hydrogen sulfide is also separated because of its corrosive character, as is the water because of the forming of hydrates which are harmful to the transportation of natural gas. The obtained natural gas is then called non condensed gas, and it mainly contains (for example 55-99% by volume) methane as well as ethane, generally propane and possibly low amounts of nitrogen and/or carbon dioxide.
Most of the natural gas is used for domestic and industrial heating; however, there are some processes for converting natural gas into higher hydrocarbons.
The direct conversion of natural gas into ethylene would be a highly desirable objective. since ethylene can serve as a raw material for numerous syntheses of important products. Both the pyrolysis and the catalytic pyrolysis of methane are both processes which aim to achieve this objective. However, these are very endothermic processes which require considerable amount of energy. Besides, these two processes produce large amounts of an desired coke.
The pyrolysis of ethane is also a well-known process which is very endothermic and thus is a large energy consumer. However, as the pyrolysis of ethane is carried out at lower temperatures than that of methane, it is not possible to simultaneously convert these two compounds. Thus, when the process is operated at the conversion temperature of ethane, the methane which is present in the ethane-containing charge comes out of the reactor essentially unchanged.
To produce ethylene and other hydrocarbons, the oxidizing coupling of methane, either in the sequential or in the simultaneous mode, has been suggested.
The reaction of the oxidizing coupling in the sequential mode consists of the oxidation of the methane by a reducing agent, followed by the re-oxidation of this agent, separately, by the oxygen in the air. Several U.S. patents (for example U.S. Pat. Nos. 4,499,323; 4,523,049; 4,547,611; 4,567,307) have mentioned the use of numerous metal oxides, mainly Mn, Ce, Br, Sn, In, Ge, Rb, Sb, Bi, Tb, as reducing agents for this reaction.
The reaction of the oxidizing coupling in the simultaneous mode (scavenging of a mixture of methane and oxygen on a contact mass) can be written qualitatively: ##STR1##
The use of rare-earth oxides, of alkaline and alkaline-earth oxides, and of titanium, zirconium, hafnium and zinc oxides, either alone or mixed, as catalysts for the reaction of the oxidizing coupling of methane in the simultaneous mode has been mentioned in several patents (for example European patents EP 210,383 A2; EP 189,079 A1; EP 206,044 A1 and world patent WO 86 07351).
It has to be noticed that the previous processes works concerning the oxidizing coupling of methane have led to the forming of low ethylene to ethane ratios, generally ranging from about 0.8 to 1.2 for C.sub.2 + product selectivities higher than about 65%. Such low ethylene to ethane ratios require the separation of ethane from ethylene and the costly pyrolysis of ethane into ethylene. Besides, the previous processes for the oxidizing coupling of methane have not taken into account the other major constituents of natural gas such as ethane, propane and other saturated hydrocarbons. The processes for the oxidizing coupling of methane described above are little applicability for the selective conversion of natural gas into olefins because of the presence of significant amounts of light hydrocarbons (such as ethane and propane) in natural gas. The relative oxidation velocities of light alkanes under the conditions of the oxidizing coupling of methane are about 15 to 100 times higher than those of the oxidation of methane, that is to say that the light hydrocarbons are converted into ethylene and carbon oxides before the methane begins to react. Thus, the previous processes cannot be efficiently applied to natural gas.
Considering the sometimes appreciable presence of propane, possibly of other light alkanes in natural gas and possibly of light hydrocarbons from other units located for example close to the natural gas oxipyrolysis unit, it is very advantageous to upgrade, besides the methane and the ethane, the propane and possibly other higher alkanes.